An isolated perfused preparation of rat hemicorpus which consists predominantly of skeletal muscle will be used in a continuation of studies on protein turnover and its hormonal and non-hormonal regulation. Studies completed thus far have shown that the perfused hemicorpus is a suitable model for investigating skeletal muscle metabolism. The preparation remains in a good physiological state during perfusion for periods up to 3 hours as judged by the maintenance of perfusion pressure, oxygen consumption, cell morphology, hormone responsiveness, and in vivo levels of high energy phosphates. Peptide- chain initiation was restricted in skeletal muscle of diabetic or 48- hour fasted rats and this step became the rate-limiting process in the pathway of protein synthesis during perfusion of preparations from normal fed animals. Addition of insulin to the perfusate relieved the block in initiation in preparations from diabetic or 48-hour fasted animals. In preparations from normal animals the hormone maintained the initial rate of protein synthesis and in vivo levels of ribosomal subunits and polysomes and decreased protein degradation. Insulin prevented a net release of amino acids due to its effects on synthesis and degradation. Pretreatment of normal rats with hydrocortisone caused the initiation block that developed during perfusion to be manifested at an earlier time and increased protein degradation. Adrenalectomy had little effect on synthesis, but slowed the rate of protein degradation. Studies are planned which include development of a cell-free system to investigate the initiation process and determination of whether the lysosomal system is involved in the effects of insulin on protein degradation.